| As an effective means to utilize distributed power,with its operation control and energy management technology,micro-grid can make full use of distributed power and promote the development of smart grid in the future.Most distributed power sources are connected to the micro-grid through the inverter.Due to its fast dynamic response,small overload capacity,low rotational inertia and low short-circuit capacity,the stability of the micro-grid will face huge challenges.The micro-grid inverter that simulates a synchronous generator(called a virtual synchronous machine)has both the characteristics of a synchronous generator and the advantages of power electronic devices.The virtual synchronous machine technology not only can improve the capacity of the micro-grid to accept new energy,but also promote China’s future smart grid construction.Based on this,the thesis studies the micro-grid offgrid operation and the control technology of seamless transfer between grid-tied and islanded operation modes.In this thesis,by analyzing the mathematical model of the synchronous generator,the ontology model of the virtual synchronous machine is established.According to the governor and excitation system of the synchronous generator,the power frequency regulator and excitation regulator of the virtual synchronous machine are designed,and the active frequency regulation and reactive voltage regulation functions of the synchronous generator are simulated.In order to improve the system’s ability to cope with nonlinearity,it is proposed to introduce a resonant controller in the voltage control link of the virtual synchronous machine control system to quantitatively analyze the influence of control parameters on the performance of the controller.In actual operation,it can be adjusted according to the system bandwidth and gain requirements;The tuning process of virtual inertia and damping parameters is provided,which lays the foundation for the subsequent theoretical research and simulation modeling.The micro-grid has two main operating modes: grid-tied operation and islanded operation.In islanded operation mode,the virtual synchronous machine needs to provide reliable power to the local load.Due to the diversity and access randomness of local load,the output voltage of virtual synchronous machine is unbalanced.In this thesis,by analyzing the mechanism of imbalance voltage,comparing the performance of vector proportional integral controller and proportional resonance controller,it is proposed to introduce vector proportional integral control into the voltage loop to achieve the output voltage balance.The control strategy does not need to separate the positive and negative sequence of the voltage,and the control structure is easy to achieveAccording to the operation situation of the micro-grid,sometimes it is necessary to switch between the two operating modes.In order to achieve seamless transfer between gridtied and islanded operation,this thesis analyzes the self-synchronization characteristics of the virtual synchronous machine,explains the process of the off-grid switching.By introducing the working principle of sine wave lock to obtain the voltage information of power grid,a pre synchronization control based on sine wave lock is proposed,which can adjust the amplitude,phase and frequency of the output voltage of virtual synchronous machine to be consistent with the power grid,and realize the micro-grid disconnection seamless transfer.Finally,the thesis builds a virtual synchronous machine simulation model,simulates the operation of the micro-grid with virtual synchronous generator technology in the gridconnected mode,off-grid mode and load changes,and verifies that the micro-grid with virtual synchronous generator technology has excellent performance;verified the effectiveness and correctness of the proposed control strategy in the process of transfer between grid-tied and islanded operation. |
